1. Field of the Invention
This invention relates to calibration methods of IC testers which perform testing of ICs to examine their quality in manufacturing. This application is based on patent application No. Hei 8-334268 filed in Japan, the content of which is incorporated herein by reference.
2. Prior Art
The IC tester is designed to perform testing of ICs (where `IC` stands for `Integrated Circuit`) to make a decision as to whether the ICs are manufactured with good quality or not. In the testing, the IC tester supplies test signals to pins of a testing IC which is subjected to testing. However, due to some factors or variations, dispersion occurs on delay times of wires of the pins of the IC. To cancel the above dispersion, a timing calibration is provided between a supply source of the test signals and the pins of the IC.
Normally, the timing calibration is realized using timing correction elements (e.g., skew ducks) which are provided between the supply source of the test signals and the pins of the IC. Herein, an amount of correction corresponding to each timing correction element is managed by a computer, for example.
FIG. 3 is a flowchart showing procedures for preparation of the testing which the conventional IC tester performs. Incidentally, the flowchart of FIG. 3 shows a flow of steps which a control unit (not shown) of the IC tester performs.
Given an instruction to start testing, the control unit makes a confirmation as to whether or not a calibration file exists in step Sa1. That is, the control unit determines whether or not calibration has been previously carried out at least one time.
The calibration file contains calibration data corresponding to timing correction values which are used when the calibration is carried out. The calibration file is written into a storage (not shown) which is equipped in the control unit.
In the aforementioned step Sa1, if the control unit detects that no calibration file exists, the control unit proceeds to step Sa4, the content of which will be described later.
On the other hand, if the control unit detects in step Sa1 that the calibration file exists, the control unit proceeds to step Sa2 to determine as to whether or not a prescribed time has elapsed after the "previous" calibration.
If the prescribed time has not elapse after the previous calibration, the control unit proceeds to step Sa5, the content of which will be described later.
If the prescribed time has elapsed after the previous calibration, the control unit proceeds to step Sa3. Herein, a temperature detector (not shown) equipped in the IC tester is used to measure temperature. So, the control unit determines, in step Sa3, whether or not a variation of temperature, which occurs after the previous calibration, is greater than a prescribed value of temperature.
If the variation of temperature after the previous calibration is not greater than the prescribed value of temperature, the control unit proceeds to step Sa5.
On the other hand, if the variation of temperature after the previous calibration is greater than the prescribed value of temperature, the control unit proceeds to step Sa4 in which calibration is performed with respect to all of the pins of the testing IC.
Thereafter, the control unit proceeds to step Sa5 in which calibration data such as the time to perform the calibration, temperature and timing correction values are stored in the calibration file to renew its content. After completion of the step Sa5, the IC tester performs testing of the IC in step Sa6.
According to the conventional calibration method of the IC tester described above, the calibration is provided to perform the timing correction with respect to all of the pins of the IC. For this reason, the calibration requires a relatively large rate in device measurement time.
In addition, a decision whether to perform the calibration greatly depends on the variation of temperature. This indicates that the conventional calibration method of the IC tester cannot cope with the timing deviation due to factors irrelevant to the variation of temperature such as the secular change and change of the hardware.
Further, the conventional calibration method is designed such that the calibration should be performed for a long time before testing of the IC even if the timing correction is not required regardless of the variation of temperature.